In recent years, micro-machine (MEMS: Micro Electro Mechanical Systems, ultra-small electrical/mechanical composite device) devices and small devices with MEMS devices assembled therein have received a remarkable attention. Fundamental characteristics of the MEMS device lie in that a driving body constructed as a mechanical structure is assembled into a part of the device and that the driving body can electrically be driven with application of Coulomb's force generated between the electrodes.
Since a micro-vibration device formed by using a micromachining technology based on a semiconductor process has merits such as a small area occupied by a device, a high Q value and capability of being integrated with other semiconductor devices, its use as an intermediate-frequency (IF) filter and a high-frequency (RF) filter in wireless communication devices has been proposed from research institutions such as Michigan State university (see cited non-patent reference 1).
FIG. 10 shows outline of a vibrator constituting a high-frequency filter, that is, micro-resonator which is described in the cited non-patent reference 1. This resonator 1 includes a semiconductor substrate 2 on which a polycrystalline silicon input side interconnection layer 4 and an output electrode 5 are formed through an insulating layer 3, a polycrystalline silicon vibrating beam, that is, so-called beam type vibrating electrode 7 being formed across a space 7 in an opposing relation to this output electrode 5. The vibrating electrode 7 crosses the output electrode 5 and is connected to the input side interconnection layer 4 in such a manner that it may be supported by anchor portions (supporting portions) 8 [8A, 8B] at both ends thereof. The vibrating electrode 7 acts as an input electrode. A gold (Au) film 9, for example, is formed at the end portion of the input side interconnection layer 4. In this resonator 1, an input terminal t1 is led out from the gold (Au) film 9 of the input side interconnection layer 4 and an output terminal t2 is led out from the output electrode 5.
In this resonator 1, in the state in which a DC bias voltage V1 is applied between the vibrating electrode 7 and the ground, a high-frequency signal S1 is supplied through the input terminal t1 to the vibrating electrode 7. More specifically, when the DC bias voltage V1 and the high-frequency signal S1 are inputted to the resonator 1 from the input terminal t1, the vibrating electrode 7 having a natural oscillation determined by a length L vibrates by electrostatic force generated between the output electrode 5 and the vibrating electrode 7. By the vibration thereof, a time change of a capacitance between the output electrode 5 and the vibrating electrode 7 and a high-frequency signal corresponding to the DC bias voltage are outputted from the output electrode 5 (accordingly, the output terminal t2). The high-frequency filter outputs a signal corresponding to a natural oscillation (resonance frequency) of the vibrating electrode 7.                [Cited non-patent reference 1]: J. R. Clark, W.-T. Hsu, and C. T.-C. Nguyen, “High-QVHF micromechanical contour-mode disk resonators, “Technical Digest, IEEE Int. Electron Devices Meeting, San Francisco, Calif., Dec. 11-13, 2000, pp. 399-402.        
Now, resonance frequencies of micro-vibrators that had been proposed and verified do not exceed 200 MHz at the maximum, so that with respect to conventional surface acoustic wave (SAW) filter or film bulk acoustic resonator (FBAR) filter at the GHz region, a high Q value which is the characteristic of the micro-vibrator cannot be provided in the GHz band frequency region.
At the present, a resonance peak of an output signal generally tends to decrease in the high frequency region, and in order to obtain satisfactory filter characteristics, it is necessary to improve an S/N (signal-to-noise ratio) of the resonance peak. According to the disk type resonator concerning the cited non-patent reference 1 of Michigan State University, a noise component of an output signal depends upon a signal which directly passes a parasitic capacitance C0 formed between the vibrating electrode 7 serving as the input electrode and the output electrode 5. On the other hand, the disk type resonator needs a DC bias voltage higher than 30V to obtain a sufficient output signal and hence it is desirable that a beam type structure using center beam should be used as a practical vibrating electrode structure.